This invention relates generally to mass spectrometry and more particularly to an apparatus and method for ion mass spectrometry that detects ions via ion cyclotron resonance.
Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS or FTMS) is a generally known instrumental method that offers higher mass resolution, greater mass resolving power, and higher mass accuracy than other currently available mass analysis methods. The principles of the FTICRMS are well described in several recent review articles and the articles referenced therein. These review articles include: A. Marshall, Milestones in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Technique Development, International Journal of Mass Spectrometry, Volume 200, 2000, pp. 331-356; Amster, I. J., Fourier Transform Mass Spectrometry, J. Mass Spectro. 1996, 31, 1325-1337; A. Sarah, E. Lorenz, P. Maziarz III, and T. Wood, Electrospray Ionization Fourier Transform Mass Spectrometry of Macromolecules: The First Decade, Applied Spectroscopy, Volume 53, No. 1, 1999, pp. 18A-36A, and A. Marshall and C. Hendrickson, Fourier Transform Ion Cyclotron Resonance Detection: Principles and Experimental Configurations, International Journal of Mass Spectrometry, Volume 215, 2002, pp. 59-75.
The performance of the FTICRMS is achieved through the combination of electric and magnetic fields, and is based upon the principle of ion cyclotron resonance (ICR). See, Lawrence, E. O.; Livingston, M. S., The cyclotron, Phys. Rev. 1932, 40, 19. Ions in the presence of a uniform static magnetic field are constrained to move in circular orbits in the plane perpendicular to the magnetic field and are unrestricted in its motion parallel to the field. The radius of this circular motion is dependent on the momentum of the ions in the plane perpendicular to the magnetic field. The frequency of the circular motion (cyclotron frequency) is a function of the mass-to-charge ratio of the ion and the magnetic field strength. Furthermore, trapping electrodes provide a static electric field, which prevent the ions from escaping along the magnetic field line. The ions are confined within the trap and as long as the vacuum is substantially high (typically  less than 10xe2x88x929 mbar), ion/neutral collisions are minimized and the ion trapping duration is maximized. Under such conditions, ions can be contained for a long period of time, which in a general mass spectrometry experiment is typically on the order of several seconds.
When the ions are initially trapped, they have an initial low amplitude cyclotron radius defined by their thermal velocity distribution and their initial radial positions. This low amplitude motion is of random initial phase, a state called xe2x80x9cincoherentxe2x80x9d oscillatory motion. While these ions are trapped, an oscillating electric field can be applied perpendicular to the magnetic field causing those ions having a cyclotron frequency equal to the frequency of the oscillating electric field to resonate. The resonant ions absorb energy from the oscillating electric field, accelerate, gain kinetic energy and move to a higher orbital radii. This process, termed xe2x80x9cion excitationxe2x80x9d, adds a large amplitude coherent cyclotron motion on top of the low initial thermal amplitude incoherent cyclotron. The net effect is that ions of a given cyclotron frequency, and hence mass, orbit as a packet. When the applied excitation field is switched off, the ions stop absorbing energy and the packet then orbits the chamber at the fundamental cyclotron frequency of the ions that make up this packet. The ion packet produces a signal by inducing onto nearby electrodes an image potential that oscillates at the same cyclotron frequency. This signal induced on the electrode can be amplified, detected, digitized, and stored in computer memory. The signal is typically in the form of a damped sine wave function with the characteristic frequency as described above. As long as the magnetic field in which ions are confined is relatively homogeneous, frequency can be measured very accurately and consequently, the mass-to-charge ratio can be measured with high accuracy.
U.S. Pat. No. 3,937,955, entitled xe2x80x9cFourier Transform Ion Cyclotron Resonance Spectroscopy Method and Apparatusxe2x80x9d, teaches a method of detecting the signal with a broadband amplifier and subsequently performing a Fourier transformation of the signal to provide a complete mass spectrum. This technique allows for acquiring and detecting all ions simultaneously and with very high mass accuracy.
U.S. Pat. No. 4,535,235, entitled xe2x80x9cApparatus and method for injection of ions into an ion cyclotron resonance cellxe2x80x9d, teaches that ions generated external of the magnet field can be injected into the ICR cell for analysis. Accordingly, prior to injecting the ions into the ICR, the ions are transmitted along an ion guide, subjected to electric fields for various functions such as mass selection and energy damping. While the ions are trapped within the ICR cell, other techniques are performed to enhance trapping and fragmentation.
It is generally known that virtually every aspect of FTICRMS performance improves at higher magnetic field. For example, if one compares a 14 Tesla magnet to the 7 Tesla instruments that are currently widely available, resolution and signal intensity will triple, mass accuracy will improve by a factor of 2, collisionally activated dissociation (CAD) fragmentation energy will increase by a factor of 4, upper m/z limit will increase by a factor of 4.
High field magnets of the type used in FTICRMS are generally electromagnets and, more specifically, due to the field strength, stability and homogeneity advantages of modern superconducting materials, they are superconducting electromagnets. Currently available superconducting magnet materials must be maintained at low temperature (variable, but typically  less than 10K) to retain their superconductivity. Therefore, these magnets are usually cooled by immersion in liquid helium (xcx9c4.2K). Due to the relatively high cost of liquid helium, this immersion vessel, called a Dewar, is then subsequently immersed in liquid nitrogen (which is much less expensive) to decrease the helium boil-off rate. New methods of cryorefrigeration as taught by U.S. Pat. No. 5,848,532, have recently been applied to greatly decrease the boil-off of liquid nitrogen and helium cryogens, and some companies now offer superconducting magnet systems that are completely cryogen free.
Applying superconducting electromagnets to the FTICRMS experiment results in some compromises between the ideal superconducting electromagnet design and the ideal FTICRMS experiment. In general, the narrower the bore size of the magnet, the easier it is to generate higher magnetic fields with sufficient homogeneity and stability for FTICRMS. However, a narrow magnet bore diameter also means that the vacuum chamber that housed the FTICRMS experiment must also be narrow thus restricting the pumping speed of the system. Typical FTICRMS vacuum chambers currently used are in the range of 100 to 150 mm representing a tradeoff between the mutually exclusive goals of achieving high magnetic field and high vacuum simultaneously with current FTICRMS designs. If one were to design a higher magnetic field system with a bore diameter sufficient to accommodated the above-indicated vacuum chamber, and with the required magnetic field homogeneity (typically  less than 10 ppm over a 5 cm diameter by 5 cm long cylindrical region), the magnet will require a larger number of windings and larger size magnets (and larger Dewar). This translates into a higher system cost and larger footprint. Since both lab space and funding are shrinking commodities, this approach, while workable, is undesirable.
Another approach of providing higher magnetic field is a reduction to the bore diameter while maintaining the number of windings and magnet size. The magnets used throughout the NMR field provide 0.1 ppm homogeneity over a 1 cm spherical volume (which is more than sufficient for FTMS), with typically 25 mm-54 mm bore diameter. If one considers installing a high vacuum system into such a diameter, pumping speed immediately becomes a serious problem because of the small throat of the bore tube. For example, a 25 mm internal diameter 0.5 m long vacuum system will have a maximum conductance (in the molecular flow regime) of 3.75 l/sec in the ideal case that ion optics, support brackets, or wires are not blocking the flow (C=12 D3/L where C is the conductance, D is the diameter and L the length of the vacuum chamber). See, Moore, J. H., Davis, C. C., Coplan, M. A., Building Scientific Apparatus: A Practical Guide to Design and Construction, 2nd ed., Perseus Books Publishing; L. L. C., Perseus, Mass., 1991. Achieving the  less than 1xc3x9710xe2x88x929 mbar pressure regime needed for ions to remain in a high amplitude, coherent cyclotron orbit becomes very difficult with a pumping speed of 3.75 l/sec. This is particularly true when the outgassing of the vacuum chamber walls is taken into consideration or when performing experiments using pulsed collision gas. For a stainless steel vacuum chamber maintaining a base pressure of 1xc3x9710xe2x88x927 mbar after one day of pumping without bake-out requires a pumping speed of 0.1 l/sec for every square centimeter of surface area. The 25 mm diameter, 0.5 m long tube has almost 800 cm2 surface area disregarding the added surface area of the cell, ion optics, wires, etc. A minimum pumping speed of 800 l/sec is required. At 1xc3x9710xe2x88x929 mbar, the required pumping speed is approximately 2 orders of magnitude higher. A pumping speed of 3.75 l/sec is generally insufficient.
In a paper entitled High-Resolution Accurate Mass Measurements of Biomolecules Using a New Electrospray Ionization Ion Cyclotron Resonance Mass Spectrometer, by B. E. Winger, S. A. Hofstadler, J. D. Bruce, H. R. Udseth, and R. D. Smith, Journal of American Society for Mass Spectrometry, Volume 4, 1993, pp. 566-577, a FTICRMS instrument was described with a cryo-panel mounted in the vacuum system for improved pumping speed. This instrument inserted a large surface area cold array (xcx9c20 Kelvin) into the room temperature high vacuum chamber inside the FTMS magnet. With this instrument, Winger et. al clearly demonstrated improved pumping speed. However the instrument used a room temperature bore magnet, room temperature vacuum system, and only the panel inside the vacuum system was cooled.
Also, a paper entitled Confinement in a Cryogenic Penning Trap of Highest Charge State Ions from EBIT, by D. Schneider, D. A. Church, B. Beinberg, J. Steiger, B. Beck, J. McDonald, E. Magee, and D. Knapp, Rev. Sci. Instrum. 65 (11), November 1994 pages 3472-3478, show the design and the use of a cryogenic electron beam ion trap (EBIT) and cryogenic penning trap (RETRAP). The EBIT""s are experimental physics instruments that are widely utilized within the trapped ion physics field. These instruments are designed to trap positive ions inside the electric field generated by high current electron beams that are collimated using a large magnetic field (several Teslas). The primary purpose for these instruments is for atomic spectroscopy measurements. The EBIT trapping mode fragments all molecules and strips the remaining atoms of electrons, for example, even to the point of producing Uranium 92+ atomic ions which are bare nuclei without any electrons. Because of this, EBITs are fundamentally limited in analysis of molecules and completely unsuitable for the analysis of intact biomolecules. However, the electron beam can be turned off, and then the positive atomic ions can be transferred to the RETRAP, where single species monitoring experiments are conducted. Ion detection is observed by a tuned circuit capable of measuring only one ions"" axial frequency at a given time, making this method unsuitable for mass spectrometry over a broad m/z range. The RETRAP uses a magnetic field generated by liquid helium cooled Helmholtz coils. The Helmholtz coil system consists of two similarly wound layered coils, spaced apart at a distance equal to the radius of the coils. This configuration has the advantage of permitting an optical access port to be mounted between the coils for conducting optical experiments. Schneider et al. suspends the magnet assembly, which includes the Helmholtz coils and the liquid cryogen, within the vacuum chamber. The vacuum chamber, which also contains the ion guide, is further submersed in a liquid nitrogen bath to help maintain the cryostat condition within the trap. This is a brute force method requiring large amounts of liquid cryogen for operation, and minimal attempts to reduce the thermal transfer between the vacuum system and superconducting magnet are evident. Additionally, because the superconducting magnet is integrated with the vacuum system, the normal operation procedures including routine maintenance and service become laborious. Access to the trap or magnet requires venting the vacuum to atmosphere, and to service the trap, the magnet must be discharged and warmed up to prevent ice formation inside the vacuum chamber or the magnet assembly. For fundamental physics research, the high cryogen consumption rate and the extraneous operation effort have been the norm. However, from a commercial approach, this design is not economically attractive.
In fundamental physics research, Penning traps, like the ones used by Schneider et al., are used to trap and detect ions"" axial motion and consequently, they are designed to maintain a hyperbolic electric field along the magnetic field axis so that the axial frequency won""t shift substantially over the oscillation. In FTICR mass spectrometry instruments, the ICR cell traps ions and are designed to detect the ions"" cyclotron motion rather than their axial motion. This function requires radial homogeneity in the electric fields and in the magnetic field.
Some of the Penning trap instruments are cryogenic as described by Winger et al., but efforts to improve magnetic field strength and homogeneity by minimizing bore diameter are not undertaken as there is little need for higher field at the mass range of atomic ions.
There have been several instruments in which a penning trap is held at very low temperatures ( less than 4.2K) have been used for high mass accuracy trapped ion mass measurement. See, G. Gabrielse, X. Fei, L. A. Orozco, R. L. Tjoelker, J. Haas, H. Kalinowsky, T. A. Trainor, and W. Kells, Cooling and Slowing of Antiprotons below 100 meV, Physics Review Letters, Volume 63, No. 13, 1989, pp. 1360-1363. In this case, the vacuum system and the bore tube of the magnet are generally held at room temperature and, a cryogenically cooled probe, with the penning trap inside, is inserted into room temperature magnet bore tube. Ions are generated by internal electron impact or an external positron source is used to generate ions that are transferred, at high kinetic energy ( greater than 1 keV, but typically  greater than 1 MeV), through a titanium window (where they lose some kinetic energy), and are trapped in the cell. Ion optics are minimal, and the penning trap is completely enclosed so that the pressure drops to  less than 1xc3x9710xe2x88x9212 mbar. Measurement of ion mass is performed using a resonant circuit to improve the accuracy of an already known mass which is not the same as mass spectrometry in which a broad range of masses are interrogated during the measurement.
In view of the forgoing, the present invention provides an apparatus for ion cyclotron resonance mass spectrometry. The apparatus has a magnet, preferably a superconducting magnet, for generating an ion confinement magnetic field within a bore of the magnet, and a vacuum chamber received inside the bore. The dimension of vacuum chamber is close to the dimension of the magnet bore, and there is preferably minimal or no thermal shielding between the magnet Dewar and the vacuum chamber to prevent thermal exchange between the magnet Dewar and the vacuum chamber. Both the magnet and the vacuum are contained within a cooling chamber such that they can be cryogenically cooled together. This allows the vacuum chamber to be cooled to a temperature close to the operating temperature of the superconducting magnet. The low temperature of the vacuum chamber during operation allows the chamber wall to function as a cryogenic vacuum pump, thereby proving enhanced vacuum in the chamber.
The present invention also provides a method of performing ion cyclotron resonance mass spectrometry. A magnet, preferably a superconducting magnet, is provided for generating an ion confinement magnetic field within a bore of the magnet, and a vacuum chamber is positioned in the bore of the magnet, preferably with minimal thermal shielding to allow heat exchange between the magnet Dewar and the vacuum chamber. Both the magnet and the vacuum chamber are placed within a cooling chamber and cooled together until the superconducting magnet reaches an operating temperature and the vacuum chamber reaches a temperature sufficiently low to provide cryopumping. Ions to be studied are then injected into vacuum chamber within the ion confinement field generated by the magnet and analyzed by means of ion cyclotron resonance.